Roof ventilation: the code numbers, how air moves through an attic, and what to ask a roofer

The numbers in the code

Virginia adopts the International Residential Code with its own changes. For houses and townhouses the roof ventilation rules sit in Section R806 of the 2021 Virginia Residential Code, read here on a code-reproduction site, so confirm against the official text. Each row below is one requirement.

Source: 2021 Virginia Residential Code, chapter 8, UpCodes copy, opened October 26, 2026.

Intake to exhaust, step by step

Picture one rafter bay in a cross section, from the lowest edge of the roof to the peak. The air path runs in this order.

1. Outside air meets the intake. Intake openings sit at the eaves or low on the roof. ARMA says continuous eave vents should run the whole length of the eave.
2. The air enters the rafter bay. A clear channel has to exist between the insulation and the underside of the roof sheathing. The code asks for at least 1 inch at the vent.
3. It travels up the slope. ARMA says to keep flow open from eave to ridge in each rafter space, and notes that skylights in enclosed rafter spaces or badly placed insulation can block it.
4. It picks up heat and water vapor. ARMA names both as the loads ventilation carries out of the attic.
5. It leaves at the ridge. Exhaust belongs at the ridge or in the upper roof. Where two different exhaust types share one attic, ARMA warns the system can short-circuit, leaving the attic incompletely ventilated.

This guide covers pitched roofs with attics. For low-slope buildings see the flat roof page, where there is no attic path to draw.

Working the 1/150 rule on paper

The arithmetic is simple. Divide the floor area of the vented space by 150 to get the minimum net free area in square feet, then split it between intake and exhaust. ARMA describes a balanced split as intake at 50 to 60 percent and exhaust at 40 to 50 percent, with intake always larger.

Vented space

1,200 square feet of attic floor (a made-up figure for the arithmetic, not a measured house).

Minimum total

1,200 divided by 150 is 8 square feet of net free area, or 1,152 square inches.

60/40 split

4.8 square feet of intake and 3.2 of exhaust. Intake is larger, as ARMA asks.

Worked arithmetic only. The measured floor area, the attic layout and the vent labels for your house decide the real numbers.

What blocks or breaks the airflow

The sources agree on a short list of causes. None of them needs a roofer to spot, but each needs someone to look in the attic.

• Insulation pushed over the soffit vents or into the rafter bays. ENERGY STAR says never to lay insulation over soffit vents.
• A kitchen, bath or dryer exhaust that ends in the attic. ENERGY STAR lists it as a warning sign, and the code forbids it.
• A dryer duct that ends too close to a ventilated soffit. The code's fallback distance is 3 feet from any building opening.
• Mixed exhaust types on one attic, which ARMA says can short-circuit it.
• Intake that is smaller than exhaust, against ARMA's balance guidance.

Signs the attic is already telling you

ENERGY STAR names these as warning signs: damp insulation, which can point to a leaky roof, moldy or rotted rafters or joists, and little or no attic ventilation. ARMA lists premature roof failure, buckled shingles, rot, wet deck or insulation, and ice dams as results of poor ventilation. EPA adds that moisture control is the key to mold control, so a cleanup without a fix for the water source is likely to see the mold return.

Winter shows the same links. ENERGY STAR lists ice dams as a symptom of attic air leaks and says air sealing combined with attic insulation can help alleviate them.

Vented or unvented attic

Not every attic is vented by design. Section R806.5 allows an unvented attic, but only when every listed condition is met. Two roofs can look alike from the street and be built under different rules.

Converting from one to the other changes the insulation, so it is a design question for the county and the designer, not a vent swap. A roofer who proposes closing the vents should name the section being followed.

What to ask any roofer to write down

A written scope that says only "install new vents" cannot be checked against the code. Turn the numbers above into specific lines for your attic:

• The measured floor area of each vented space and the minimum net free area worked out from it.
• The net free area of the intake and exhaust products named, from their labels.
• The intake to exhaust split, with intake larger than exhaust.
• Which vents in the field of the roof are being replaced, and which soffit or gable vents are staying and how they will be checked.
• How the 1-inch channel above the insulation will be kept open, and who restores it if insulation is touched.
• Where every bath, kitchen and dryer duct ends, and that none ends in the attic.
• The code section followed if the attic is to be unvented.

ARMA says vents in the field of the roof get replaced during a re-roof, while soffit and gable vents can stay if they work afterward. More on structuring a scope is in our roof replacement guide, and repairs are covered on the roof repair page.

Where Arlington fits

Ventilation is part of the roof assembly, so it rides along with whatever permit or review the job already needs. The Arlington roofing hub sets out the permit table, the building-type test and local historic district review. Condo and co-op buildings add their own deciders, covered in the condo and co-op guide. Owners of older houses can start with the subdivision roof guide.

For metal roofs see the metal roof page, and for the full list of materials the materials overview. Roofers of Arlington installs, repairs and replaces roofs, flat and metal systems included, in Arlington and nearby. If you want to talk through the attic and the roof above it, book a phone consultation. Seasonal checks are in the winter roof maintenance post, and a general walk-through in the roof inspection post.